Fusion draw processes such as described in U.S. Pat. Nos. 3,338,696 and 3,682,609, issued to Dockerty, herein incorporated by reference, are used to form sheet materials with pristine surfaces of fire-polished quality. A typical fusion draw process involves delivering molten material into a contoured channel of a weir device. The molten material wells over the weir device, divides, and flows down the sides of the weir device. At the bottom of the weir device, the divided flow converges to form a single sheet with pristine surfaces of fire-polished quality. The single sheet is translated through a series of heated zones and drawn into a final desired thickness.
Fusion draw processes can deliver sheet materials that do not require post-forming finishing operations such as lapping and polishing for advanced applications such as flat panel displays. However, it can be difficult to control stress level and variation within the sheet while drawing the sheet, and high stress level and variation within the drawn sheet may result in permanent distortion in the final product. One factor that contributes to stress and stress variation within the sheet is sheet motion. Typically, the sheet is only physically constrained at or near the edges while in the draw in order to maximize the quality area of the sheet. As a result, the bulk of the sheet is free to move while the sheet is being drawn. Such sheet motion has been found to negatively impact the level of stress and stress variation within the sheet. Processes performed at the bottom of the draw that involve applying forces to the sheet, such as sheet separation processes, can also exacerbate sheet motion. Other factors that contribute to high stress level and stress variation within the sheet include coarse temperature control in the annealing zone of the fusion draw machine, or inadequate inline annealing, and uncontrolled air currents from chimney effect within the fusion draw machine.